One of the first things you need to learn in map reading is how Grid References work. A grid reference is a small code that relates to a place on the map. Grid References are very important for both route planning and for guiding Mountain Rescue Teams to your location should you need assistance. This guide deals with the Irish Grid System used by Ordnance Survey Ireland (OSI)
NOTE: Grid References are different to Latitude and Longitude which are international reference points. OSI has a handy online coordinate converter.
What is the grid system?
The island of Ireland has been divided into a 5×5 grid labelled A-Z (the letter I is not used to avoid confusion with the letter L). Each box in the grid represents a 100km x 100km area.
Each of these boxes is further divided using vertical and horizontal grid lines each 1 kilometre apart. The vertical lines are numbered 1 to 99 going west to east. The horizontal lines are numbered 1 to 99 going south to north.
Four Figured Grid Reference
The first thing we’ll learn is how to get a four-figure grid reference. We’ll use the summit of Carrauntoohil as the example.
There are three parts – Letter, Easting and Northing (Think LEN for short)
- Take the letter for the current area. On Ordnance Survey maps, the letters are displayed on the map in blue. In our example, this is V
- Next, we go “across the corridor” and figure out the Easting. This is the number of the grid line to the left of the box. In our example, this is 80
- Finally, we go “up the stairs” and get the Northing. This is the number of the grid line at the bottom of the box. In our example, this is 84
This will give us the grid reference V 80 84
Six Figured Grid Reference
Our course, a 1000 x 1000 meter area is rather large. Therefore, it’s more common for us to use a six-figure grid reference. We start by working out the four-figure grid reference as above. We divide the box into an imaginary 10×10 grid.
For the easting, we count inwards from the left to the exact point we’re looking at. We add this number to the grid reference. In our example, the summit is 300 meters from the left of the box. This makes the full easting 803
Likewise, for the Northing, we count upward from the bottom and add this number to the grid reference. In our example, the summit is 400 meters from the bottom of the box making the northing 844
This will give us the grid reference V 803 844
Several compasses include a “roamer” to make calculating six-figure grid references easier. It works by placing the corner of the roamer at the point in the map and counting the dashes from there to the gridline. In the image below, you’ll see the Silva Expedition 4 compass which has 3 Roamers for different scale maps (1:25,000, 1:40,000 and 1:50,000)
Key grid references
Let’s look at some other grid references from around the country
Mount Brandon, Kerry – Q 460 116
Lugnaquilla, Wicklow – T 032 917
Galtymore, Tipperary/Limerick – R 878 238
Slieve Donard, Down – J 357 276
Here are some handy resources if you want to learn more
Map Scales and Grids – Ordnance Survey Ireland
Grid References 1 – Mountaineering Ireland
Grid References 2 – Mountaineering Ireland
Grid References – Scouting Ireland
For more guides on map and compass skills, check out the rest of our Navigation 101 series. Let us know in the comments if we missed anything.